1. Field of the Invention
The present invention relates to the insertion of codes or binary identification or authentication messages in digital images.
2. Discussion of the Related Art
A first example of application relates to the insertion of an identifier of the person entitled to the rights relative to the image, to enable detection of a possible piracy by distribution of illicit copies, or even going back to its source. A second example of application is the protection against set-ups deforming or falsifying the image, to guarantee the integrity of an image. The technique of binary code insertion in digital images is generally known as “watermarking” and has had a recent expansion with the rapid development of digital image transmission, especially over the Internet. Considering the increasing risk of illicit copies, the persons entitled to royalties, in particular, have searched for means to protect digital multimedia content.
The two above examples concern so-called security applications. Non-security applications may also be found, for example, the inserting of characteristics linked to the shooting or other information about the image.
Generally, the insertion of a code concerns each image. In the case of an animated sequence, all images then includes an insertion (for example, an identification code of the author is reproduced in all images). In this example, it is besides possible for the content of the respective codes to be linked to the sequence, to detect a possible image insertion or suppression.
Whatever the application, a so-called marking algorithm is used to encrypt and position in the image the codes to be inserted therein. An inverse algorithm is used to restore, according to the application, the identification, authentication codes, or the shooting characteristics, etc.
Conventionally, marking algorithms modify either certain pixels in the image, or certain coefficients in a transform of the image (Fourier transform, discrete cosine transform, etc.). The pixels or coefficients to be modified are generally selected by using a key specific to the user (the coder). The modifications may be, for example, an inversion of coefficient couples so that their values respect a relation depending on the value of the bit to be hidden, or a mere addition of the message to be hidden, previously modulated by Gaussian white noise.
The modification or coding often takes the form of an addition of a specific noise forming the hidden message. It is accordingly difficult to predict the influence of the modifications on the final image which may even, in certain cases, be visibly altered.
U.S. Pat. No. B1-6,181,802 discloses a method for marking digital images consisting in localizing the regions in which the information are coded at the level of the image contours. The shapes of the contours are not modified; they are not deformed. In '802, the pixels adjacent to the contour are modified for coding the message in the image.
Another disadvantage of conventional approaches is that they do not enable finding the hidden message again once the image has undergone some manipulations (modifications). For example, if the pixels are displaced with respect to one another (geometric manipulation of rotation type, for example) or if they are modified by filtering, compression, etc., there is a significant risk of loosing the encrypted message. Among the most frequent manipulations which are likely to prevent giving back messages, the recentering, a JPEG-type compression before transmission of the image in digital format, or a contrast, luminosity, or image size modification should be mentioned.
The above problem is even more critical when the image undergoes several successive manipulations, which is more and more often the case with image transmissions over digital communication networks.